The present invention relates to a method for the preparation of phenyl pyruvic acid or, more particularly, to a method for the synthetic preparation of phenyl pyruvic acid by the reaction of benzyl chloride and carbon monoxide in the presence of a basic compound of an alkaline earth metal and a cobalt carbonyl compound as the catalyst.
As is known, phenyl pyruvic acid is an organic compound useful as a starting material for the synthesis of various kinds of compounds including phenyl alanine, which is a useful compound as an intermediate for the synthesis of, fcr example, an artificial sweetening agent, and others.
Among various synthetic routes for the preparation of phenyl pyruvic acid, the industrially most promising method is the reaction of benzyl chloride and carbon monoxide. It is proposed, for example, in Japanese Patent Publication No. 56-18587 corresponding to U.S. Pat. No. 4,152,352 that the reaction of benzyl chloride and carbon monoxide is catalyzed by a metal carbonyl compound or, preferably, a cobalt carbonyl as the catalyst and the reaction is performed in a binary solvent mixture of water and alcohol in the presence of a basic compound of an alkaline earth metal.
In the conventional methods for the preparation of phenyl pyruvic acid including the above mentioned proposal in the Japanese patent, the desired compound can readily be obtained by the treatment of the precipitates in the form of an alkali or alkaline earth metal salt of the acid, which is precipitated in the reaction mixture after completion of the reaction and recovered by filtration or other suitable means for solid-liquid separation, with an acid. A difficult problem, however, is encountered in these prior art methods in connection with the mutual separation and recovery of the phenyl acetic acid formed as a by-product in a considerable amount in the form of an alkali or alkaline earth metal salt and the cobalt carbonyl catalyst since both of the by-product and the catalyst are dissolved in the filtrate after recovery of the phenyl pyruvate. Moreover, the cobalt constituent, if separated from the filtrate solution, cannot be used as such for the catalytic purpose in the next run of the reaction without a very elaborate and troublesome procedure for the regeneration of the cobalt carbonyl catalyst.
To explain the regeneration procedure of the cobalt catalyst from the filtrate of the reaction mixture, the solvents, i.e. water and alcohol, are first removed from the solution by evaporation and the residue is treated with an inorganic acid to isolate the by-product phenyl acetic acid. The salt of cobalt with the inorganic acid is then converted into cobalt hydroxide by the treatment, for example, with an alkali hydroxide followed by the carbonylation reaction of the hydroxide with water gas into the cobalt carbonyl compound under a high pressure and at a high temperature. Thus, it is eagerly desired to reduce the costs for the cobalt catalyst.
Another problem in the above mentioned method as proposed in the Japanese patent is that the reaction must be performed under a pressurized condition of 5 to 200 bars or, preferably, at least 40 bars of the pressure in order to obtain an industrially practicable yield of the product. At least, the yield of the desired compound is quite low when the reaction is undertaken under normal pressure. Needless to say, a great advantage would be obtained if the reaction can be performed under normal pressure to give a satisfactorily high yield of the product.